If you draw out the structure of the hydroxonium ion H3O+, you will see that the central oxygen has 8 electrons in its outer shell while each hydrogen does not have any other electrons except the shared pair (which is part of the oxygen octet).

(Original post by charco)
If you draw out the structure of the hydroxonium ion H3O+, you will see that the central oxygen has 8 electrons in its outer shell while each hydrogen does not have any other electrons except the shared pair (which is part of the oxygen octet).